Vacuum deposition of coatings is in widespread use today, and appears to be of growing importance in the future. Cathode sputtering induced by glow discharges is emerging as one of the more important processes for effecting such depositions. Much of the recent work relates to various magnetron geometries in which enhanced sputtering rates and operation at lower pressures are achieved through judicious use of magnetic fields. An extensive literature has developed and many patents have issued over the past decade. A particularly informative and reasonably current summary is contained in the book "Thin Film Processes" edited by John L. Vossen and Werner Kern, published by Academic Press, New York, 1978. Particularly relevant chapters are: Chapter II-1, "Glow Discharge Sputter Deposition", by J. L. Vossen and J. J. Cuomo; Chapter II-2, "Cylindrical Magnetron Sputtering", by John A. Thornton and Alan S. Penfold; Chapter II-3, "The Sputter and S-Gun Magnetrons", by David B. Fraser; and Chapter II-4, "Planar Magnetron Sputtering" by Robert K. Waits.
In order for the intensity of glow discharges to be enhanced through the application of magnetic fields, it is necessary that electrode geometries, magnetic field intensities, and magnetic field geometries be selected in such a way as to produce electron traps. In most cases, crossed electric and magnetic fields give rise to electron drift currents which close on themselves. In the case of cylindrical magnetrons, for example, radial electron traps can be formed with essentially uniform magnetic fields parallel to the axes of the cathode and anode cylinders. By providing the cathode with electron reflecting surfaces at its ends, loss of electrons from the discharge through axial drift can be reduced, thus further enhancing the discharge intensity, and making operation at lower gas pressures possible. (See, for example, above-referenced Chapter II-2, "Cylindrical Magnetron Sputtering", by John A. Thornton and Alan S. Penfold, especially pp. 77-88.)
In many of the magnetrons employed commercially for sputter deposition, electron trapping is accomplished by shaping the magnetic field relative to the shape of the sputter target (cathode). In particular, most planar magnetrons employ a magnetic field which loops through the planar cathode surface and which forms a tunnel-shaped magnetic field which closes on itself. (See, for example, above-referenced Chapter II-4, "Planar Magnetron Sputtering", by Robert K. Waits, especially page 132.) Under normal operating conditions, the glow discharge is largely confined to this magnetic tunnel.
Magnetic tunnels are also employed with non-planar magnetron configurations. An example of a hollow cathode cylindrical magnetron employing a single magnetic tunnel is shown in FIG. 4, p. 118 of above-referenced Chapter II-3, "The Sputter and S-Gun Magnetrons", by David B. Fraser. In addition, examples of cylindrical magnetrons employing multiple magnetic tunnels are shown in FIG. 3., p. 78 of above-referenced Chapter II-2.
Another circular magnetron sputter source in commercial use employs a cathode (target) of a generally inverted conical configuration surrounding an axially symmetric central anode. An example of such a sputter source may be found described in more detail in U.S. Pat. No. 4,100,055, issued July 11, 1978 to Robert M. Rainey and entitled "Target Profile for Sputtering Apparatus" and assigned to the assignee of the present invention. Such a sputter source is also commercially available from and manufactured by Varian Associates, Inc. under the trademark "S-Gun". This type of sputter source is also described, for example, in above-referenced Chapter II-3, especially FIG. 1, p. 116 and FIG. 3, p. 117. In particular, FIG. 3, p. 117 shows schematically the magnetic field looping through the conical cathode (target) surface to form a magnetic tunnel which confines the glow discharge.
In prior art magnetic tunnels, the energetic electrons which sustain the glow discharge would need to cross magnetic field lines to escape from the magnetic tunnel, which they are unable to do if the magnetic field intensities are great enough. Also, those electrons which have been captured into the discharge are energetically incapable of reaching the cathode. Thus, even though these electrons may follow magnetic field lines toward the cathode surface, they will be electrostatically reflected from the cathode surface back into the discharge.
If the magnetic field intensity falls off with distance from the cathode surface, as it does in most prior art magnetic tunnels, "magnetic mirroring" can also contribute to electron reflection. The main effect of such magnetic mirroring is, on average, to move the region of electron reflection a bit further from the cathode surface. This effect is incidental rather than crucial to the magnetic tunnel's role in reflecting electrons in order to contain the glow discharge. In any event, those electrons which would otherwise escape through the magnetic mirror will be reflected electrostatically back into the discharge. It is therefore both convenient and proper to refer to the electron reflection in the prior art simply as "electrostatic", even though some magnetic mirroring may be occurring.
Discharge intensity tends to be a maximum in the center of a magnetic tunnel, where the magnetic field lines are generally parallel to the cathode surface, and falls off rapidly as the sides of the magnetic tunnel are approached. Localized cathode (target) erosion rates correspond generally with the immediately adjacent intensity of the glow discharge, thus leading to nonuniform erosion of the cathode surface. Examples of nonuniform erosion of an S-Gun cathode are shown in FIG. 3 of above-referenced U.S. Pat. No. 4,100,055 to Rainey, and examples in the case of planar magnetron cathodes are shown in FIG. 5, p. 141 of above-referenced Chapter II-4.
One consequence of nonuniform cathode erosion is that there is less-than-maximum utilization of target material. Another consequence of nonuniform cathode erosion is that changes may occur in the distribution pattern of sputtered material leaving the cathode surface. Additionally, the glow discharge tends to move downward in the magnetic tunnel to maintain close proximity to the cathode surface as the cathode surface erodes away. This movement when coupled with nonuniform cathode erosion tends to concentrate the discharge even more sharply, leading to still greater nonuniformity of cathode erosion. Furthermore, such nonuniform cathode erosion restricts the area of emission of sputtered atoms to a relatively narrow band on the cathode surface. This in turn restricts the range of direction of sputtered atoms arriving at the substrate to be coated, thus affecting such film properties as uniformity and step coverage, both of which are of particular importance in metalization of semiconductor wafers, for example. Also, the deposition rate from a deeply eroded cathode may be reduced because of geometrical shielding effects. In addition, nonuniform cathode erosion is attended by correspondingly nonuniform cathode heating, which contributes adversely both to cathode cooling problems and to thermal stressing of the cathode.
Using prior art magnetic tunnels, a further consequence of the movement of the glow discharge with erosion of the cathode surface is that the discharge generally moves into a region of greater magnetic field intensity. This results in a lowering of the discharge impedance, which requires lower operating voltage, higher dishcarge current, and higher discharge power to maintain a fixed deposition rate (see above-referenced Chapter II-2, pp. 94-98; also see above-referenced Chapter II-3, pp. 117-121). An indication of the severity of this problem in some applications is conveyed by U.S. Pat. No. 4,166,783 issued Sept. 4, 1979 to Frederick T. Turner and entitled "Deposition Rate Regulation by Computer Control of Sputtering Systems" and assigned to the assignee of the present invention.
Accordingly, it is an object of the invention to provide a glow discharge sputter source in which input power can be maintained constant throughout cathode (target) life for constant deposition rate.
Another object of the invention is to provide a sputter source which operates with higher sputtering efficiency, whereby power consumption and power supply size are reduced.
Another object of the invention is to provide a sputter source in which the electrical impedance of the glow discharge remains substantially constant throughout cathode (target) life, whereby the problems of supplying and controlling power are reduced.
A further object of the invention is to increase the utilization of target material, thereby increasing target life.
Yet another object of the invention is to maintain a more uniform distribution pattern of sputtered material leaving the cathode surface over the useful life of the cathode.
A further object of the invention is to increase the width of the band from which sputtered atoms are emitted from the cathode, whereby coatings having improved properties may be obtained.
A still further object of the invention is to ease the cathode cooling problem, thereby allowing operation at higher powers and at correspondingly greater sputter deposition rates.
Yet another object of the invention is to reduce thermal stressing of the cathode, whereby fracture, localized melting, and the like are avoided.
A further object of the invention is to provide an improved means for holding the cathode in place, whereby brittle and weak cathodes may be retained without breakage caused by the holding means.
A still further object is to reduce the amount of unused target material in the cathode.